1. Field of the Invention
Embodiments of the present invention generally relate to analog and digital discriminators, and more specifically to apparatuses and methods for minimizing the effects of dark-current pulses from Silicon Photomultipliers.
2. Description of the Related Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones and/or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones and/or tissues of interest. For example, the radiopharmaceutical (e.g., rubidium) is injected into the bloodstream.
The radiopharmaceutical produces gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of events (e.g., a single gamma when using Single Photon Emission Computed Tomography (“SPECT”) and coincident gamma events when using Positron Emission Tomography (“PET”)) allows an image of the organ or tissue under study to be displayed.
FIG. 1 depicts a prior art system 100 that includes a known apparatus 102 for superposed magnetic resonance (“MR”) and PET imaging. The apparatus 102 includes a known MR tube 104. The MR tube 104 defines a longitudinal direction Z (not shown), parallel to a longitudinal axis of a patient (also not shown), which extends orthogonally with respect to the plane of the drawing in FIG. 1.
As shown in FIG. 1, a plurality of PET detector units 106 arranged in pairs opposite each other about the longitudinal direction z is arranged coaxially within the PET scanner 104. The PET detector units 106 preferably include a silicon photo-multiplier (“SiPM”) array 108.
A computer 120 is also included in the system 100. The computer 120 includes a central processing unit (“CPU”) 114 for image processing of superimposed MR and PET images, a user interface 118 (depicted as a keyboard), and a monitor 116 for viewing input and output data.
The prior art SiPM array 108 is depicted in FIG. 2 and consists of an n×n array of pixels 202. For illustrative purposes the n×n pixel array 202 is depicted as a 5×5 pixel array. The Pixels in the array 202 are identified by an (x,y) index. For example, pixels along the “X” axis are denoted as 202(1,1), 202(1,2), . . . , and 202(1,5); and pixels along the “Y” axis are denoted as 202(2,1), 202(2,1), . . . , and 202(5,1).
Typically, each individual pixel is an m×m array of parallel microcells 204, with each microcell consisting of a reverse biased avalanche photodiodes (“APD”) 2061, . . . , 206m and an active or passive quenching mechanism such as resistors 2081, . . . , 208m (collectively 206 and 208, respectively). Parasitic capacitance across the quench resistor 208 is also present.
The APD 206 is reverse biased to a voltage Vb, greater than the breakdown voltage Vbrk. When a photon is absorbed in the junction, it can cause a photo-electron to be released. The photo-electron drifts to a region of high electric field where it accelerates and causes additional electrons to be released by impact ionization. During this breakdown, current flows through device as the junction discharges to the breakdown voltage. At this point, the junction recovers and again begins to function as reverse biased diode. During the following recovery phase, current flowing through the device charges the junction back to the bias voltage Vb. During a complete breakdown and recovery cycle, the net amount of charge released from the microcell is:Q=(Cd+Cq)(Vb−Vbrk)  Equation 1
where Cd is the diode junction capacitance, and Cq is the parasitic capacitance across the quench resistor.
A scintillator (not shown) attached to the SiPM sensor 108 converts a high energy gamma-ray to many photons. The photons cause multiple cells to breakdown. Since the cells of an SiPM sensor 108 are biased beyond Vbrk, they also breakdown randomly at a high rate due to thermal effects, causing dark-current pulses.
Since dark pulses appear identical to the first photoelectron from a true event, the high rate of dark-pulses can limit the performance of the discriminator. In some cases, a small subset of microcells may contribute to most of the dark-pulses. In this case, others reduce the effect of noisy microcells by deactivating them, requiring a unique look-up table for each SiPM sensor. However, this is not an ideal solution since the remaining microcells still create dark pulses, and it is cumbersome to have a unique look-up table for each SiPM sensor.
There is a need in the art for a design that does not unnecessarily deactivate pixels to reduce false triggers due to dark pulses.